The liquid crystal display device has been used as a display device for many electronic devices such as a mobile phone, a digital still camera, a personal digital assistant, and a medical equipment. Such a liquid crystal display device includes a backlight irradiating the liquid crystal display panel with an illumination light. A typical structure of the backlight employs, for example, as described in Japanese Unexamined Patent Publication No. 2006-98500, a configuration including a light guide plate and a reflection sheet on the back side of the liquid crystal display panel and arranged with a light-emitting diode serving as a light source on the side portion of the light guide plate.
On the other hand, as shown in FIG. 1, the liquid crystal display device equipped with a large backlight for a medical application and the like employs a configuration arranged with a light-emitting diode substrate 6 mounted with a light-emitting diode 7 on the side portion of the backlight. The liquid crystal display device shown in FIG. 1 includes a liquid crystal display panel 1 accommodated in an upper frame 4 and fixed by a mold 3 via a cushion 2, and a backlight constituted by a light guide plate 9 accommodated in a lower frame 5 and irradiating the back side of the liquid crystal display panel 1 with the illumination light, an optical member 10 including a diffusion sheet, prism sheet, and the like for irradiating the liquid crystal display panel 1 uniformly with the light guided by the light guide plate 9, a reflection member 8, and a light-emitting diode 7 irradiating the light guide plate 9 with the illumination light from the side portion of the light guide plate 9, wherein the light-emitting diode 7 is mounted on the light-emitting diode substrate 6 and accommodate and arranged in a recess 12 protruding on the back side of the lower frame 5.
In the liquid crystal display device shown in FIG. 1, the light emitted from the light-emitting diode 7 enters through a side face of the light guide plate 9 and diffuses all over the light guide plate 9. The light about to exit from the back side of the light guide plate 9 is reflected by the reflection member 8 to be returned to the inside of the light guide plate 9, and the light exiting from the front side of the light guide plate 9 is controlled by the optical member 10 constituted by the diffusion sheet, the prism sheet, and the like to fall on the entire liquid crystal display panel 1 uniformly.
The lower frame 5 provided with the recess 12 for accommodating and arranging the light-emitting diode substrate 6 is generally formed of a light metal such as aluminum or a resin. To form the recess 12 in the lower frame 5, the U-shaped recess 12 as shown in FIG. 1 is formed by a mechanical bending process or the like. Due to the limited ability of the process, it is inevitable that the convex portion has a substantial width.
The light guide plate 9 and the reflection member 8 need to be arranged as close to the light-emitting diode 7 as possible to propagate and reflect the light emitted from the light-emitting diode 7 without a loss. Accordingly, the light guide plate 9 and the reflection member 8 in the proximity of the light-emitting diode 7 partially protrude in a space formed by the recess 12 toward the light-emitting diode 7.
As a result, the portion of the reflection member 8 protruding into the space formed by the recess 12 bows downward in the recess 12 over time due to its own weight, forming a gap in between with the light guide plate 9. When the gap is formed between the reflection member 8 and the light guide plate 9, as indicated by a dashed arrow in FIG. 1, a part of the light emitted from the light-emitting diode 7 and falling downward from the light guide plate 9 cannot be reflected upward by the reflection member 8 but diffused to the outside. Consequently, an availability of the light emitted from the light-emitting diode 7 reduces and the brightness lowers on the whole backlight or locally. Such a gap between the reflection member 8 and the light guide plate 9 is also generated by a deformation due to the stress applied when forming the recess 12 in the lower frame 5 or by a warpage of the lower frame 5 due to aging.
Another problem arising when the light-emitting diode 7 is used as the light source of the liquid crystal display device would be heating of the light-emitting diode 7. That is, the luminous efficiency of the light-emitting diode is typically between a few percent and twenty percent, with a large part of the remaining energy being converted into heat leading to heat generation. This heat generation may cause a characteristic degradation of the light-emitting diode and peripheral devices.